Absorption of neutral amino acids by the gut ofArenicola marina

Summary The absorption of neutral amino acids byArenicola marina was studied using anin vitro preparation of the alimentary canal. Regional variation in absorption was observed, with the intestine being the region of greatest uptake. The L enantiomorphs of the neutral amino acids alanine and leucine were shown to be actively absorbed by the intestine as was the D enantiomorph of alanine. A saturable component was demonstrated in the absorption of L-alanine and this was shared by L-methionine, which was found to competitively inhibit alanine uptake. Inhibition of L-alanine uptake also occurred in the presence of other neutral, basic and acidic amino acids. The greatest inhibition was found with the L stereoisomers of methionine, leucine, valine, histidine and phenylalanine, whilst proline, lysine and aspartic acid decreased uptake to a smaller extent.     

Export of amino acids to the phloem in relation to N supply in wheat

The effect of different N supply on amino acid export to the phloem was studied in young plants of wheat (Triticum aestivum L. cv. Klein Chamaco), using the exudation in EDTA technique. Plants were grown in a growth cabinet in pots with sand, and supplied with nutrient solutions of different NO₃^? concentrations. When plants were grown for 15 days with nutrient solutions containing 1.0, 3.0, 5.0, 10.0, 15.0 or 20.0 mM KNO₃, the exudation rate of sugars from the phloem was unaffected by N supply, but sugars accumulated in the leaf tissue when the N supply was limiting for growth. On the other hand, the rate of exudation of amino acids was proportional to the NO₃^? concentration in the nutrient solution. When the supply of N to plants grown for 15 days with 15.0 mM NO₃^? was interrupted, the exudation of sugars was again unaffected, but there was a fast decrease in the amount of amino acids exudated, and of the concentration of amino acids and nitrogen in the tissues. Also, when 10-day-old plants grown without N were supplied with 15.0 mM NO₃^?, there was a sharp increase in the exudation of amino acids, without changes in the amount of sugar exudated. The rate of exudation of amino acids to the phloem was independent of the concentration of free amino acids in the leaves in all three types of experiment. Asp was the most abundant amino acid in the leaf tissue, while Glu was the one most abundant in the phloem exudate. Asp and Ala were exported to the phloem at a rate lower than expected from their leaf tissue concentrations, indicating some discrimination. On the contrary, Glu showed a preferential export at low N supply. It is concluded that the rate of amino acid export from the leaf to the phloem is dependent on the N available to the plant. This N is used for synthesis of leaf protein when the supply is low, exported to the phloem when supply is adequate, and accumulated in the storage pool when supply is above plant demand.     

Phospholipid methylation and the synaptosomal uptake of mediator amino acids

The decrease in neurotransmitter amino acid uptake was observed in rat brain synaptosomes incubated with S-adenosyl-L-methyl-methionine. The inhibitory effect of neurotransmitter as a consequence of methylation of synaptic membrane is more pronounced in stimulatory transmitter amino acids. The effect of phospholipids on amino acid uptake in rat brain synaptosomes decreases with age.     

Floral gradient in flowering tobacco in relation to free amino acids

By employing TCLs (thin cell layers) culture, the floral gradient in flowering tobacco of different developmental stages was confirmed. The TCLs from early flowering tobacco regenerated more floral buds than those from the tobacco plants in full blooming or fruiting stages. Analysis of free amino acid levels revealed the acropetal gradient of Pro in flowering tobacco stem. L-Pro. L-Trp. D,L-Met and L-Arg were respectively added into the culture medium for testing their influence on floral bud formation from tobacco pedicel segments. Only L-Trp evidently enhanced the floral bud neoformation.     

Hepatic uptake of amino acids at mid-lactation in the rat.

The enzymic determination of D-3-hydroxybutyrate and acetoacetate normally involves the use of 3-hydroxybutyrate dehydrogenase (HBDH, EC 1.1.1.30) of bacterial origin. We show that HBDH from Rhodopseudomonas spheroides (BCL, grade II) contains a 3-hydroxyisobutyrate dehydrogenase (HIBDH) activity: activity with 3-hydroxyisobutyrate as substrate was greater than 10% of that with 3-hydroxybutyrate. However, HBDH could be prepared essentially free of HIBDH activity by incubation at 37 degrees C in the presence of 1 mM-CaCl2, to produce an enzyme preparation that may be used for the specific determination of 3-hydroxybutyrate. Use of the purified enzyme preparations indicated that a major product of valine metabolism in hemidiaphragms from 40 h-starved rats was 3-hydroxyisobutyrate rather than 3-hydroxybutyrate.     

Blue-light-control of the uptake of amino acids and of ammonia in Chlorella mutants

In non-photosynthetic, yellow or colourless mutant cells of Chlorella kessleri , grown with nitrate as sole nitrogen source, blue light inhibited the uptake of the amino acids glycine, proline and arginine and of ammonia in growing cells, while it enhanced the uptake of these amino acids in resting cells. On the other hand, in cells grown with ammonia as the only nitrogen source without nitrate reductase activity, blue light did not influence the uptake of amino acids and of ammonia in growing cells, while it enhanced the uptake of amino acids in resting cells. Addition of methionine sulphoximine, a potent inhibitor of glutamine synthetase, to growing cells, resulted in intracellular ammonia-accumulation and inhibition of uptake of glycine and of ammonia. For the colourless mutant, blue light was shown to activate purified nitrate reductase. These results indicate that in the mutant cells of Chlorella examined, uptake of ammonia seems to be influenced by nitrate reductase and the uptake of amino acids was influenced by both nitrate reductase and an unknown blue-light-receptor(s). The uptake of urea in mutant cells is not influenced by the irradiation with blue light. Uptake of glycine was also increased after addition of glucose (hexose) in the dark. Because blue light is known to enhance the breakdown of starch, a reaction producing glucose for oxidative degradation in the algae used, the role of glucose (hexose) in the blue light-affected uptake of amino acids is discussed.     

Differential microbial uptake of dissolved amino acids and amino sugars in surface waters of the Atlantic Ocean

Nitrogen bioavailability is considered to limit the productivityof oceanic oligotrophic gyres, the largest biomes on Earth.In order to assess the microbial requirement for small organicnitrogen molecules in these and other waters, the microbialuptake rates of amino acids (leucine, methionine and tyrosine)and amino sugars (glucosamine and N-acetyl-glucosamine) as wellas glucose were compared using a bioassay technique of radiotracerdilution. The bioassays were carried out on four mid-Atlanticmeridional transects spanning a latitudinal range from 60°Nto 42°S. The mean concentrations of both bioavailable N-acetyl-glucosamineand glucose in the gyres were 1 nM, four times higher than themean leucine concentration. Despite its lower concentration,the mean turnover time of leucine in the gyres of 15 h was 90and 9 times shorter than the turnover time of N-acetyl-glucosamineand glucose, respectively. In addition, among amino acids, leucinewas taken up in the gyres at a rate of 1.5 times faster thanmethionine and 2.5 times faster than tyrosine. Hence, oceanicbacterioplankton as a community showed a clear preference foramino acids, particularly leucine, compared with amino sugars.The preferential uptake of amino acids to sugars challengesthe concept of microbial nitrogen or carbon limitation in theopen ocean.     

Uterine uptake of amino acids and placental glutamine--glutamate balance in the pregnant ewe

The uterine uptake of amino acids was studied in 10 pregnant sheep with gestational ages of 114-146 days. After recovery from surgery, arterial and uterine venous samples were drawn simultaneously via indwelling catheters and analysed for amino acid and oxygen content. In seven ewes, amino acid concentrations were measured by a chromatographic technique. In four ewes, glutamate and glutamine arterio-venous differences across the uterine and umbilical circulations were measured by an enzymatic method. The uptake of neutral and basic amino acids was 66 mumol/mmol O2 and 17.3 mumol/mmol O2, respectively. Comparison of uterine and umbilical uptake shows that the bulk of the neutral and basic amino acids taken up by the pregnant uterus are transferred to the fetus. there was no significant uptake of acidic amino acids (i.e. glutamate, aspartate and taurine). glutamate was delivered from the fetus to the placenta but excretion of glutamate into the uterine circulation was negligible. Glutamine and asparagine were delivered to the fetus in amount which were two to three times larger than the placental uptake of glutamate and aspartate. Therefore placental conversion of exogenous glutamate and aspartate to glutamine and asparagine cannot account entirely for the fetal uptake of these amino acids.